If you’re assuming the best way to drive conversion is to put big "call to action" buttons above the fold, you’re missing out on the more important point of placing actions where people become convinced to act (this is key).

If you're assuming ads above the fold create the most brand presence or impact, you may be confusing view-ability with actual viewing.

There are design/development techniques that allow you to reposition content based on screen size. So if you do want elements visible on a screen regardless of its size, you can. This is “fold independent” layout.